Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)
Credit: Text-book of embryology / edited by Walter Heape. Source: Wellcome Collection.
Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.
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![The must widespread ulterabiou iu the conditious ol' the larva wliich is met with is its trausformatioii into an embryo tln-ougli its retention within the egg-shell or the mother's body. Since, as a race progresses from point to point in evolution, it should, according to the biogenetic law, leave behind it a trail of larval stages, eacli corresponding to a condition of life wliich had formerly been the adult one, and in each of which the organism would have a distinct method of obtaining its food and a special set of enemies, a very long and complicated life-history should be ]jroduced. But the dangers of such a long larval life are very great, therefore a great advantage would be obtained by passing over some of these stages within shelter, and, as was pointed out above, in all life-histories we find an embryonic stage at the beginning. Now the food necessary for development during the embryonic phase is, in the, vast majority of cases, furnished in the form of yolk platelets, embedded in the cytoplasm. This yolk sometimes distends the embryonic cells to enormous proportions; it renders the process of cell-division difficult, and sometimes even impossible. In the ordinary process of cell-division each daughter nucleus becomes at, and immediately after the time of nuclear division, the centre of an attractive force which tends to collect the cytoplasm round it like a ball. In some cases, in consequence of this force, the first products of the division of the egg appear as spheres toucMng one another only at a point. But this period of activity is succeeded by a period of quiescence, and the centripetal force subsides, so that the cytoplasm of the two daughter cells tends to flow together again, unless a cell membrane has been formed between them in the meantime. When yolk is present it impedes the action of the centripetal force, apparently by rendering the cytoplasm less viscous; for cytoplasm devoid of yolk behaves like tluck honey, whilst that wMch is loaded with yolk behaves more like a mixture of honey and water. Consequently, in yolky eggs we find that in the first stage of their development they either divide into a few large clumsy cells, or else that cell division is represented by nuclear division only. Further, since the yollv is never uniformly distributed in the ovum, but is usually massed at one side, the first divisions result in the produc- tion of cells of unequal sizes or in the production of a cap of cells at one side of the egg, the rest remaining unsegmented. The pole of the surface of the egg wliich is relatively freer Irom I'ood-yolk, and where the division into cells first occurs, is termed the animal pole of the egg; it is here too that the polar bodies are given off. The opposite pole, where most of the yolk is accumulated, is termed the vegetative pole. Such eggs are termed meroblastic, whilst eggs in which the yolk is sufficiently small in amount to allow of the division of the whole are said to be holoblastic. Further, whilst in most eggs the yolk is massed at one side (telolecithal) {:vide Fig. 3 a), in a wide range of eggs it is massed at the centre, surrounded by a rind of comparatively yolk-](https://iiif.wellcomecollection.org/image/b21506589_0001_0062.jp2/full/800%2C/0/default.jpg)
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